The invention relates to an apparatus for fractional distillation under vacuum and more particularly, to an apparatus for film-type distillation under vacuum, also called molecular distillation. In film-type distillation the liquid to be distilled is caused to flow along an evaporator surface in a relatively thin film for partially vaporizing the liquid, the vaporized liquid is collected on a cooling surface for condensing said vapor to form liquid distillate. The distillate and residue, i.e., non-vaporized liquid, are collected separately and withdrawn from the apparatus.
The above type of distillation is particularly suitable for distilling heavy liquids with extremely high boiling points, and is normally conducted at pressures of only a few microns Hg absolute. This is a high vacuum compared with conventional vacuum distillation units operating at a few mm Hg absolute at least. Molecular distillation might therefore advantageously be applied for distilling residues from conventional vacuum distillation units. The known molecular distillation units, however, are less suitable for distilling the large quantities of liquid to be processed in the petrochemical and oil industry. Further, the known units are often too complicated for use in large, continuously operating refineries.